The fabrication of efficient sails for sailboats, that is, providing sails which are capable of being controlled to provide sufficient aerodynamic "lift" to allow the boat to sail to windward, while being conveniently handled in use and stowed when not in use, is relatively complex. In order to be useful by the vast majority of sailors, sails must be made of lightweight flexible membranes of various natural and synthetic, woven and composite "sailcloth" materials, so that the sails can be folded, rolled, carried by hand, and stored in relatively small bags, while the sails must also be capable of being shaped by relatively simple control mechanisms to assume efficient aerodynamic shapes varying with wind conditions and the boat's course relative to the wind.
More particularly, in order to make effective progress to windward, that is, towards the source of the prevailing wind, a sail for a sailboat must provide aerodynamic lift, such that a resultant force is exerted on the hull of the boat in a direction less than 90.degree. to the wind. Modern sailing rigs allow properly sailed boats to make progress at 30-45.degree. with respect to the apparent wind, i.e., to the wind as measured on the boat itself; normally this translates to progress at approximately 45.degree. to the "true" wind. Accordingly, in order to make upwind progress, sailboats must "tack", that is, travel in a zig-zag path, alternately at .+-.45.degree. to the direction of the true wind. The requirement of tacking in turn necessitates that the airfoils provided by the sails be readily reconfigurable in either orientation with respect to the wind, that is, so as to be readily reconfigured to provide efficiently-shaped airfoils for upwind sailing on either tack.
For this reason, and others to be discussed below, the lessons found in such diverse fields as airplane and race car aerodynamics, while instructive, cannot be directly applied to sailboat sails. More particularly, airplane and race car wings provide lift according to the same laws of physics as do sailboat sails. However, airplane and race car wings are invariably two-dimensional in cross-section, i.e., have rigid upper and lower surfaces spaced from one another by some distance; sailboat sails (with a very few exceptions mentioned below), are essentially membranes, comprising a single flexible member supported by more or less rigid spars, to take a desired shape when exposed to the wind.
Further, as mentioned above, sailboats must be able to provide maximum lift to windward while on either tack, i.e., regardless which side of the membrane faces the direction of the wind. Therefore, as a sailboat tacks to go to windward, its sails must be readily reconfigurable between mirror-imaged airfoil shapes, as the opposite surfaces of the sails alternatingly become the windward and leeward surfaces.
By comparison, an airplane wing need provide maximum lift only while taking off. In that case, the wing is always oriented in the same direction, and may accordingly be asymmetric in cross-section. Stated differently, while an inverted airplane wing may provide sufficient lift to allow the airplane to fly upside down, under no circumstances need the inverted wing provide sufficient lift to allow the airplane to take off. Similarly, modern racing cars use inverted wings to provide additional aerodynamic "downforce", i.e., lift directed toward the road surface; there is no requirement on such wings to be invertible in an analogue to tacking a sailboat.
While sailing craft having rigid "wing" sails including separately controllable elements to allow tacking to windward have been successful from the point of view of ultimate sailing performance, perhaps most notably in the successful 1988 America's Cup boat Stars & Stripes, such rigid wing sails are totally unsuited for the vast majority of sailboats, which require that the sails be readily removable and stowable. As a practical matter, therefore, sails for sailboats must be made of relatively flexible cloth materials, stiffened if at all by removable "battens", so as to be foldable and rollable for convenient removal and stowage. The "sailcloth" itself may be a woven polyester fabric, e.g. "Dacron", a laminated composite material comprising strands of high-modulus carbon or aramid ("Kevlar") fiber bonded between sheets of polyester film material "Mylar"), or various others. The teachings of the present invention are applicable to all types of cloth sails.
The typical "sail trimming" problem faced by the sailor in sailing to windward is in shaping his or her sail, and orienting it properly with respect to the apparent wind, so as to optimize the lift to aerodynamic drag ("L/D") ratio. Improvement in lift, or reduction of drag, both result in increased force driving the boat to windward. Correct sail trim also allows the boat to make progress to windward at a course closer to the direction of the true wind, that is, to "point higher", than possible if the trim is incorrect. Each of these improvements result in improved net velocity toward the source of the wind, normally expressed as velocity made good, or "VMG".
Achieving the correct sail trim is not simply a matter of setting one's sails in the same manner whenever sailing. Rather, the correct sail trim varies widely with wind speed, wave conditions, and the boat's heading with respect to the wind. Accordingly, modern sails are cut so as to respond to a wide variety of adjustment devices provided by the corresponding sailboats. For example, in sailing to windward, the L/D ratios of sails are commonly optimized in response to apparent wind speed by adjusting the "depth" or fullness of the sail. In general, at relatively low wind speeds, the sail is adjusted to be relatively full, increasing lift at some cost in aerodynamic drag; as the wind increases, the sail is flattened to reduce drag, adequate lift being provided by the increase in wind speed. At some wind speed, the lateral forces exerted on the hull of the boat by the sails may exceed the boat's ability to resist such forces; it then becomes necessary to reduce sail area, typically by "reefing" the sail.
It will be appreciated by those of skill in the art that VMG can be improved by providing increased lift, and/or reduced drag, particularly at low wind speeds, where lift is at a premium, preferably together with improvements in a given sail's pointing ability; each of these improvements results in increased VMG, and if provided together would provide a substantial increase. However, doing so will only be useful if the use of the sail is not otherwise interfered with; for example, one could always increase lift at a given wind speed by providing a larger sail, but this is impractical for existing boats, as larger and more expensive spars, stronger rigging and the like would be required. A larger sail will also need to be reefed earlier unless concomitant increases are made in the boat's sail-carrying ability, e.g., by addition of ballast. In any event, the rules of many racing classes improvement of the performance of racing sailboats being a principal aspect of the invention--prohibit carrying larger sails.
It would, however, be appropriate to provide improvements to the design of sails, specifically to provide sails of a given size but exhibiting improved L/D ratio and/or improved pointing ability, that could be implemented without interfering with the normal functioning of the sails, e.g., without interfering with the normal hoisting, lowering, reefing, removal, and stowage procedures. Further, it is desired that any such improvements be suitable for implementation by way of ready and inexpensive retrofit modifications to existing sails, and without requiring significant modification to the spars, rigging or hull of the boat. It is also desired that any such modifications not significantly complicate the usual sail trimming and handling procedures, such that the steps involved in carrying out routine sailing manuevers, e.g., tacking, are not complicated further.